There are three typical systems for ink-jet printing. In one system ink droplets are ejected through a small nozzle normally from several microns to several hundreds microns in diameter, and each droplet is imparted a predetermined static charge by electrical signals, then deflected electrostatically between high voltage deflection plates to form a character or figure on a recording medium according to a predetermined dot matrix. In a second system ink droplets are drawn through a small nozzle of the above specified size under the electrostatic control of high-voltage plates, accelerated, and then electrostatically deflected between deflection plates to form a letter or figure on a recording medium according to a predetermined dot matrix. In the third system, ink droplets are ejected in response to electrical signals to form a letter, figure or a picture on a recording medium according to a predetermined dot matrix.
It is important that ink used in ink-jet printing not clog the small nozzle, and that it not form a solid deposit on the inner wall of the nozzle causing a change in the rate of ink injection. These requirements must be met throughout an extended continuous operation of the ink-jet printer, and they must also be met when the operation is restarted after a short or long suspension.
Other important requirements are that the ink not spread on the recording medium and that it provide a light-fast record. This means the recorded image must not dissolve in water and that it must not be discolored by light.
A still further requirement is that ink capable of achieving faithful color reproduction must be used for providing a color image by ink-jet printing.
We have found that whether a particular aqueous ink meets these requirements depends greatly upon the dye used in that ink.